The portion of the telephone network which connects each individual subscriber to a central office serving that subscriber uses a two-wire subscriber loop. At the central office, the incoming and outgoing signals are transmitted on a four-wire circuit. The subscriber telephone, the two-wire connecting pair, and a hybrid coil at the central office form a balanced bridge circuit such that the hybrid coil separates the incoming and outgoing signals at the two- to four-wire conversion point.
An effective impedance imbalance of the bridge circuit can cause a substantial portion of the remote speaker's signal to be returned to him or her in the form of an echo. The "source" of the echo is the hybrid itself, although other echo sources also can exist. The distance between the remote speaker and the bridge circuit is proportional to the round-trip delay time and also proportional to the effects of the echo. Where satellite links form a part of the communication path, for example, the effect of the echo can be severe.
The effects of echo can be reduced using an echo suppressor or an echo canceller. An echo suppressor is effective for circuits of up to a few thousand miles in length. The echo suppressor detects which direction of the conversation is active at any given time and inserts attenuation in the opposite direction, thus breaking the round-trip path and attenuating the echo.
An echo canceller is more effective for longer circuits. One type of echo canceller consists of a transversal filter connected between the two paths. Using the incoming signal as input, the filter taps are adjusted to replicate the portion of the signal that leaks through the hybrid coil. The replicated signal is then subtracted from the outgoing signal, effectively canceling the echo. Commonly, echo cancellers apply a least-mean-squares (LMS) filter adaptation algorithm to estimate the effects of the echo.
Where multiple communication channels are supported, prior-art systems use a Digital Signal Processor (ASP) with associated memory and support circuitry to support signal processing, including echo cancellation, for each channel. Commercial products that provide echo canceling functions include the AT&T Echo Canceller (Universal), Tellabs Echo Canceller (2531A), Coherent Echo Canceller (EC-6000), and the DSC Echo Canceller (EC-24). Prior-art echo cancellers have long delays because they use DSPs to perform echo cancellation and, often, to perform multiple other signal processing functions. Thus, because of clock limitations, prior-art echo cancellers typically cannot perform all calculations in a direct sequence real-time manner, and must buffer data to accommodate periodic increases in processing demands without losing data As communication systems have become increasingly digital, these component delays, which result from algorithm and implementation inefficiency, as well as limitations of vendor DSP chip sets have increasingly become a problem
What is needed is a method and apparatus which solves echo cancellation algorithm and implementation efficiency problems of the prior art Further needed is a method and apparatus which does not require dedicated component resources for echo cancellation on every communication channel. Further needed is a method and apparatus which provides full featured echo cancellation with a very short delay in the communication path.